The present invention relates to a liquid crystal display apparatus used in timepiece, portable telephone, audio, electronic apparatus and the like and relates to a constitution of increasing brightness of a liquid crystal display apparatus. In details, the invention relates to a liquid crystal display apparatus capable of carrying out both displays of a reflection type display utilizing external light which is light of an environment of use and a transmission type display utilizing illuminating light of back light or the like.
A liquid crystal panel (LCD) used in a liquid crystal display apparatus generally uses liquid crystals of a TN (twisted nematic) type or an STN (super-twisted nematic) type and is constructed by two sheets of substrates opposed to each other by interposing a liquid crystal layer in which liquid crystal molecules are twisted to align as a principal constitution. Further, polarizers are respectively arranged on a front face side and a back face side of the liquid crystal panel. According to the liquid crystal display apparatus having such a constitution, operation principle of display is established by bringing about a change in an aligning state or phase of the liquid crystal molecules by an electric field, current or temperature rise to thereby change optical properties of interference, scattering, diffraction, optical rotation, selective scattering, adsorption of light or the like in a liquid crystal state. Further, display is realized by controlling a liquid crystal layer by applying voltage between electrodes provided at the respective substrates for forming pixels.
Further, since the liquid crystal panel is nonluminous, a reflector or aback light is generally used. There is a liquid crystal display apparatus for carrying out both display modes of a reflection type display utilizing external light of natural light, interior light or the like and a transmission type display utilizing illuminating light from a back light such that display can be observed both in a bright place and in a dark place. As a constitution of such a liquid crystal display apparatus, there is generally known a constitution including a transflective reflector having a function of transmitting a portion of incident light and reflecting other portion thereof on a back side of a liquid crystal panel and a back light which is an illumination source. Such a constitution is called as a transflective display apparatus.
A transflective type liquid crystal display apparatus of a related art will be explained in reference to FIG. 10. Both sides of a liquid crystal panel 1 are provided with an upper polarizer 2 and a lower polarizer 3 and a back light 5 which is an illuminating apparatus is provided on a back side of the lower polarizer via a transflective reflecting plate 30. The back light 5 is provided with a light guide plate 7, a light source 6 provided at a side face of the light guide plate 7, and a reflecting layer 20 as illustrated.
In the liquid crystal display apparatus having such a constitution, when the liquid crystal panel 1 is displayed by utilizing external light, a light incident on the upper polarizer 2 transmits through the liquid crystal panel 1, and the lower polarizer 3 and a portion of light thereof is reflected by the transflective reflector 30. The light reflected by the transflective reflector 30 reaches an observer by transmitting through the lower polarizer 3, the liquid crystal panel 1 and the upper polarizer 2 again. Thereby, information displayed on the liquid crystal panel 1 can be seen. Next, an explanation will be given of a case of displaying the liquid crystal panel 1 by utilizing light of the back light. A portion of illuminating light illuminated by the back light 5 transmits through the transflective reflector 30. The transmitted light is incident on the liquid crystal panel 1 from a back face thereof by further transmitting through the lower polarizer 3 and is emitted to a front side of the display panel. The emitted light reaches the observer by transmitting through the upper polarizer 2. Thereby, information displayed on the liquid crystal panel 1 can be seen.
However, according to the transflective reflector used in the transflective type liquid display apparatus of the related art, when a rate of reflection is increased, a rate of transmission is reduced, conversely, when the rate of transmission is increased, the rate of reflection is reduced. Therefore, in the case of increasing the rate of reflection, when external light is utilized, the display becomes bright, however, when the back light is used, the display becomes dark. Conversely, in the case of increasing the rate of transmission, when the back light is utilized, the display becomes bright, however, when external light is used, the display becomes dark. In this way, according to the liquid crystal display apparatus using the transflective reflector, there poses a problem that the display cannot become bright in both of the external light and the back light. Further, since the transflective reflector and the reflector on the back side of the back light are used, constituent members are thickened and cause to increase the cost. Hence, it is an object of the invention to realize a constitution capable of carrying out bright display by increasing an efficiency of utilizing light from a light source by a compact constitution. Further, it is an object of the invention to realize a liquid crystal display apparatus of a two faces display type having a high efficiency of utilizing light from a light source.